A 150 kVA transformer with a primary rating of 8000 V would be additive or subtractive?

• A. Additive
• B. Subtractive
• C. Cannot determine
• D. Both

Answer: (A)

Polarity of transformer windings determines whether voltages add or oppose when units are connected together. Additive means the voltages at corresponding terminals are in phase and reinforce each other, so when two identical transformers are paralleled, their currents share and the overall capacity sums. Subtractive means the voltages oppose each other, causing circulating currents and poor sharing.

For a transformer with an 8 kV primary, this type of unit is typically intended to be paralleled in a distribution system. The winding connections and polarity marks are arranged so that, when paralleled, the voltages are in phase and add in effect, giving additive behavior. Subtractive would lead to unwanted circulating currents and ineffective sharing, which is avoided by design.

So the correct concept is additive.